My Book Office Edition 1TB Disappeared on Yosemite

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On such cases it is recommended to use Mac OS’s built-in Disk Utility in order to test and diagnose the hard drive. There are times when the file system is corrupted or in need of repair before Finder can mount/access the storage volume. I would also recommend changing the USB cable:

http://wdc.custhelp.com/app/answers/detail/a_id/866/